I think Toei just wasn't sure what to do with the Sixth Ranger dynamic at the time. They probably thought the kid Rangers would appeal to their young audience. It is odd there was no Sixth in Kukuranyer or Carranger.

agreed with you with Kakuranger and while there wasn't a 6th Ranger in Carranger they technically considered Signalman as the team's official 6th member and I think Ninjaman was supposed to represent the 6th Kakuranger member as well